In certain cases, when an aircraft is on the ground in an airport parking area for instance, the aircraft needs to be moved. For example, it can be necessary to move an aircraft if it has broken down, or to move the aircraft out of the parking area, or to reverse it, etc.
It is known to use a tow vehicle, also called a tractor, which tows the aircraft to move it to a specific location, to move an aircraft on the ground. During these towing phases, the aircraft is pulled or pushed by a tow vehicle connected to the aircraft's front landing gear, more simply called the nose gear.
An aircraft's nose gear can be oriented to the right or the left to allow the aircraft's movement along curves to the right or left, with respect to a longitudinal axis of the aircraft. On certain aircraft this nose gear orientation is obtained by means of a steering system. This steering system comprises a hydraulic system associated to a command prepared by a computer, from pilot inputs via an interface. The power made available to the steering system is supplied by the hydraulic system, which can also power other systems of the aircraft. This steering system can be either activated or deactivated.
When the steering system is activated the nose gear is controlled by the pilot via a pilot control unit installed in the aircraft cockpit. When the steering system is deactivated the nose gear is free, i.e. it is no longer controlled and can therefore follow the movements imposed by an external element, e.g. towing by a vehicle.
Generally, activating and deactivating the steering system is controlled by means of a control device located outside the aircraft.
The control device for the nose gear steering system can be, for instance, a lever or any other control located on the gear itself or near to the gear. This control device, subsequently called lever, comprises an electrical switch that, in an active position (or closed position), allows the nose gear steering system to be activated and, in a non-active position (open position), allows said steering system to be deactivated.
When the tractor tows the aircraft, it is necessary for the steering control to be inactive, i.e. that the steering system is deactivated, such that the nose gear is free in rotation. In effect, if the nose gear control is active, i.e. if the steering system is not deactivated, then said steering system seeks to slave the gear to its neutral position, or initial position, i.e. a position where its angle of rotation is zero. As a result, when a tractor tows the aircraft, the steering system tries to counter the movements imposed by the tractor, in particular in bends, which forces the gear's steering system and can lead to certain elements of the aircraft being damaged. The damaged elements can be structural parts of the nose gear or a hydraulic element of the nose gear steering system or any other part used in the nose gear control chain.
In certain aircraft, the gear's steering system is activated or deactivated by an operator on the ground who places the lever forming the control device in an open or closed position. This lever can be mounted on a unit fitted on the aircraft's nose gear, or close to the nose gear.
In operation without towing the lever is in the closed position, thus allowing the pilot to control the gear's steering system. The lever is kept in its closed position for the entire period of the aircraft's flight and for the entire period when the aircraft moves under its own power on the ground.
When towing must be carried out, the operator on the ground must move the lever to its open position in order to deactivate the gear's steering system. The change in the steering system's status (activated or deactivated) is controlled solely by the operator on the ground. As a safety measure, some control devices comprise a locking element that must be put in place in order to lock the control device in its open or closed position. For example, this locking element is a pin inserted into a slot to make sure that the lever remains in its open or closed position.
However, if the operator on the ground forgets to change the lever's position, if the lever remains blocked in its closed position, or if the operator forgets to lock the lever by means of the safety pin and the lever returns to its closed position during the towing phase, then the gear's steering system operates as if it was activated, which gives rise to risks of the nose gear or steering system being damaged, as explained earlier. Whatever the reason, reactivation or non-deactivation of the gear's steering system, the fact that said steering system is in an activated mode when the aircraft is being towed can be the cause of significant damage to the aircraft.